Hertz's ink jet techniques are disclosed in U.S. Pat. Nos. 3,416,153 and 3,916,421 and provide high resolution and high precision. With Hertz's on-off modulation technique, an ink jet is continuously forced out from a nozzle and broken into minute drops or droplets in flight. The nozzle is internally provided with an electrode for charging the ink, while a deflection electric field is set up between the nozzle and a record medium. Accordingly, when the voltage source for charging the ink is turned on and off, a row of charged droplets is removed by the deflection electric field, permitting a series of uncharged droplets only to proceed toward the record medium. The series of droplets impinges on a point on the record medium to form a dot.
When a computer output or the like is to be printed out, each character is expressed by selectively marking some of a plurality of pixels (picture elements) which are divided in the form of a matrix.
Journal of Applied Photographic Engineering, Vol. 5, No. 4, Fall 1979, pages 248 to 250, "Ink Jet Color Graphics" introduces a method of modulating the density of dots constituting one pixel in order to produce tones with use of Hertz's technique. With reference to FIG. 1 illustrating this method, the pixel is constituted in the form of a submatrix, and the number of dots to be marked within the submatrix of definite area is changed for the modulation of dot density. This method will hereinafter be referred to as dot pattern modulation. FIG. 1 shows dot patterns provided by 4.times.4 dot-distributed submatrices. The dot patterns, including a submatrix with no dot, produce tones in 17 steps.
When such method of dot pattern modulation is to be used for a wider range of tone reproduction with a high resolution, the total number of dots to be recorded as well as the dot density greatly increases. For example, when images are recorded on A4-size paper in 64 tones with a resolution of 5 pixels/mm, a total of about 10.sup.8 (210.times.5.times.297.times.5.times.64) dots need to be recorded at a dot density of 30(5.times.8) dots/mm. Such increases of dot number and density reduce the velocity of recording.
For the high-speed recording of images with a high resolution and high degree of tone reproduction, Proceedings of 14th Joint Conference on Image Technology, 8-1(1983), pages 185 to 188 proposes the combination of the multi-level dither method, a technique for tone reproduction, and a method of varying the size of ink jet droplets. The ink jet device used in this proposal is of the on-demand type wherein the nozzle is provided with a piezoelectric crystal, and pressure is applied to eject a jet of droplets only when recording. The droplet size is modulated by varying the voltage to be applied to the piezoelectric crystal. The proposal has the following drawbacks. One of them is that because the droplet size per se is varied, the viscous resistance of air to which the droplets are subjected in flight produces displaced dots on the record medium. As another drawback, the device is difficult to control in giving different droplet sizes with good stability. It is also difficult to produce individually stabilized droplets.